1. Field
The present invention relates generally to data communication, and more specifically to techniques for generating pseudo-random number (PN) sequences or vectors based on ROM(s).
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on, for a number of users. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access technique. CDMA systems may provide certain advantages over other types of system such as increased capacity. A CDMA system may be designed to implement one or more CDMA standards such as IS-95, IS-2000, IS-856, W-CDMA, and so on, all of which are well known in the art.
In a CDMA system, data to be transmitted over the air is spectrally spread over the entire system bandwidth to combat deleterious transmission effects such as fading and multipath. At a transmitter, the spectral spreading is performed by multiplying the data to be transmitted with a pseudo-random number (PN) sequence. The spread data is further processed to generate a modulated signal, which is then transmitted over the air.
Due to artifacts in the propagation environment (e.g., building, natural structures, and so on), the modulated signal transmitted from a given transmitter may reach a receiver via a number of signal paths. The received signal at the receiver may thus include a number of multipath components (or signal instances), each of which corresponds to the signal received via a respective propagation path. Since the receiver may receive signals from multiple transmitters, the received signal may thus include a number of multipath components for a number of transmitters.
To recover a given multipath component in the received signal, a complementary spectral despreading operation is performed at the receiver. The received signal is initially conditioned and digitized to provide samples, and the samples are then despread with a (complex-conjugated) PN sequence corresponding to the one used at the transmitter. To recover the multipath component, the PN sequence used at the receiver needs to be aligned in time with the time of the arrival of the multipath component. Thus, the phase (or offset) of the PN sequence used at the receiver to recover the multipath component is dependent on both the offset of the PN sequence used at the transmitter and the propagation delay of the multipath component.
For a CDMA system, a rake receiver is often used to concurrently process a number of multipath components in the received signal. The rake receiver typically includes one or more searcher elements (or simply “searchers”) and a number of demodulation elements (often referred to as “fingers”). Each searcher may be operated to process the received signal to search for strong multipath components. Each finger may then be assigned to process a multipath component of sufficient strength to recover the transmitted data.
Each searcher despreads the samples with PN sequences at various offsets, with each PN offset corresponding to a hypothesis being evaluated. In particular, to search for strong multipath components, each searcher typically performs a number of correlations of the samples with PN sequences at various offsets. Each correlation results in a high value if the PN sequence used for despreading is time-aligned with that of a multipath component, and a low value otherwise.
Each finger similarly despreads the samples with a PN sequence at a particular offset. This PN offset, which is dependent in part on the arrival time of the multipath component, may be initially determined by the searcher and thereafter tracked by the finger.
Conventionally, a dedicated PN generator is provided for each finger and used to generate the PN sequence at the desired offset. Similarly, a dedicated PN vector generator is provided for each searcher and used to generate the necessary PN vectors. (A PN vector is a short (e.g., 8-chip) segment of a PN sequence.) This conventional design includes a number of individual PN generators for a number of fingers and searchers. The total circuitry for all such PN generators may comprise a relatively large portion of the integrated circuit for the rake receiver. For this design, the PN generator circuitry grows as the number of searchers and fingers in the receiver increases.
There is therefore a need in the art for techniques for generating PN sequences or vectors for the searchers and fingers of a rake receiver, and which may be implemented with reduced circuitry.